In a tire pressure drop detecting apparatus of the electromagnetic induction type, the resonator assembly mounted on a tire-and-wheel assembly or, particularly, the wheel rim forming part of the tire-and-wheel assembly comprises a pressure-sensitive switch unit which is arranged to be responsive to the pressure of the compressed air in the tire chamber and which is electrically connected between a resonant inductor and a resonant capacitor. The resonance circuit thus constituted by the resonant inductor and capacitor electrically connected together across the pressure-sensitive switch unit is responsive to the electromagnetic wave radiant from the oscillator assembly when the tire-and-wheel assembly rotating about the center axis of the wheel rim assumes a predetermined angular position with respect to the oscillator assembly which is held stationary with respect to the vehicle body structure. In response to the electromagnetic wave emitted from the oscillator assembly, an electric current is induced in the resonance circuit if the pressure-sensitive switch unit intervening between the resonant inductor and capacitor constituting the resonance circuit is closed. The pressure-sensitive switch unit is adapted to be closed in response to an air pressure higher than a predetermined acceptable value and to be open in response to an air pressure lower than the predetermined acceptable value. When the pressure of air in the tire-and-wheel assembly carrying the resonator assembly is higher than the predetermined acceptable value, a current is produced in the resonator assembly each time the tire-and-wheel assembly makes a full turn about the center axis of the wheel rim during cruising of the vehicle. The induction of a current in the resonator assembly results in a normal decrement in the energy of the electromagnetic wave radiant from the oscillator assembly, which thus produces a signal representative of such a normal decrement. The signal produced by the oscillator assembly is delivered to a signal processing unit connected to suitable output means such as a display or warning device and holds the display or warning device inoperative. In the event the pressure of the air in the tire chamber drops unusually due to, for example, a puncture of the tire, the pressure-sensitive switch unit incorporated in the resonator assembly is made open so that the resonant inductor and capacitor of the resonator assembly are electrically disconnected from each other and are disabled from producing a current in response to the electromagnetic wave radiant from the oscillator assembly. As a consequence, the oscillator assembly produces a signal indicative of the elimination of a normal decrement in the energy of the electromagnetic wave from the oscillator assembly and causes the signal processing unit to actuate the display or warning device to produce a sound or luminant signal.
In a prior-art resonator assembly for use in a tire pressure drop detecting apparatus of the electromagnetic induction type such as, for example, the resonator assembly taught in Japanese Utility Model Publication No. 52-30553, the pressure-sensitive switch unit has a pressure-acting chamber communicating with the tire chamber in a tire-and-wheel assembly and a reference-pressure chamber hermetically isolated from the pressure-acting chamber by means of a flexible diaphragm of rubber or metal and having established therein a predetermined air pressure lower than a normal pressure of air in the tire chamber of a tire-and-wheel assembly. The flexible diaphragm intervening between the pressure-acting and reference-pressure chambers is biased to be deformed in a direction to contract and expand the pressure-acting and reference-pressure chambers, respectively, by, for example, the elasticity of the diaphragm per se. When the pressure of air developed in the pressure-acting chamber is higher than a predetermined value, the diaphragm is forced to be deformed in the opposite direction against the biasing force and the force constantly exerted on the diaphragm by the pressure of the air confined in the reference-pressure chamber. In the event a puncture takes place in the tire forming part of the tire-and-wheel assembly arranged with such a pressure-sensitive switch unit, there is caused a sudden drop in the pressure of air in the pressure-acting chamber of the switch unit so that the diaphragm of the switch unit is allowed to be deformed in the direction in which the diaphragm is biased to be deformed. Such deformation of the diaphragm is converted into or used as a switching action to cause the switch unit to open.
The prior-art resonator assembly incorporating the pressure-sensitive switch unit of the above described nature is mounted in its entirety on the wheel rim of the tire-and-wheel assembly and is, when in use, subjected to temperatures of a range which is as broad as to span from approximately minus 40.degree. C. to approximately 120.degree. C. If, therefore, the switch unit is enclosed in an air-tight casing structure which is hermetically sealed from the atmosphere, the pressure of the air confined in the reference-pressure chamber of the switch unit is subject to change with the change in the temperature transferred to the switch unit. The reference pressure for the switch unit being thus variable with temperature, the switch unit can not be accurately responsive to a predetermined value of the air pressure in the tire chamber.
If, furthermore, the casing structure of the pressure-sensitive switch unit of the prior-art resonator assembly is sealed from the atmosphere by the use of an organic adhesive compound applied to the contact surfaces of the component parts of the casing structure, the adhesive compound tends to generate gas therefrom when the casing structure is subjected to high temperatures. The gas thus generated from the adhesive compound is allowed into the reference-pressure chamber of the switch unit and adds to the pressure of air in the reference-pressure chamber, giving rise to an increase in the reference pressure for the switch unit.
The pressure-acting and reference-pressure chambers of the switch unit are hermetically isolated from each other by the flexible diaphragm intervening therebetween. During use of the resonator assembly, however, the pressure of air introduced into the pressure-acting chamber from the tire chamber of the tire-and-wheel assembly is inevitably allowed to leak into the reference-pressure chamber. If the casing structure of the switch unit is perfectly sealed from the atmosphere, the leak of the compressed air into the reference-pressure chamber is accumulated in a long run and also gives rise to an increase in the reference pressure for the switch unit.
The present invention contemplates elimination of the drawbacks which have thus been inherent in a prior-art pressure-sensitive switch unit of the described character.
It is, accordingly, an important object of the present invention to provide an improved pressure-sensitive switch unit in which the pressure of air in the reference-pressure chamber is at all times maintained at a constant value without respect to change in the temperature transferred to or surrounding the switch unit.
It is another important object of the present invention to provide an improved pressure-sensitive switch unit which is durable and reliable in operation when incorporated into a resonator assembly for use in an apparatus for detecting a pressure drop in a tire-and-wheel assembly.
It is still another important object of the present invention to provide a resonator assembly including the improved pressure-sensitive switch unit.
Yet, it is still another important object of the present invention to provide a tire pressure drop detecting apparatus including such a resonator assembly.